First time I took our dog, Drew, to the vet he 'buicked' all over me. Needless to say, I was pretty embarrassed and unhappy.
Dr. Posey (bar none the best vet I've ever met) said, don't feed him, don't let him in the front seat, and don't give water before car rides.
Eventually before he died, Drew got to the point where he went almost everywhere with us. We very seldom left him in the car unless it was just for a short run into someplace (like the ice cream parlor) where we could watch him. He was a pretty smart dog and he ended up always moving to the front seat of the car and sitting in the drivers seat with his paws on the wheel at the 10 and 2 position. Sometimes, he'd even honk the horn.
Hopefully the advice or the laughter will help!